Fuel injection systems, for example gasoline direct injection systems, have, in simplified terms, a high-pressure fuel pump, by means of which a fuel is highly pressurized, and a high-pressure region with a high-pressure accumulator, the so-called rail, and with at least one injector valve for injecting the highly pressurized fuel into an associated combustion chamber of an internal combustion engine. The stated components are connected to one another by means of high-pressure lines.
For the operation of the fuel injection system, a control device, the so-called ECU, with corresponding software is normally provided. By means of the control device, it is for example possible for the delivery power of the high-pressure fuel pump to be adapted. For this purpose, on the high-pressure fuel pump, for example, there is situated a valve, which may be formed for example as a so-called digital inlet valve. Said digital inlet valve may for example be provided in a “currentless open” embodiment, that is to say open when electrically deenergized, though other embodiments are also possible and known. Furthermore, for the regulation of the injection pressure required at the injector valves, a high-pressure sensor is situated in the fuel injection system, which high-pressure sensor is normally attached to the high-pressure accumulator and serves for acquiring the so-called system pressure. In the case of gasoline as fuel, said system pressure typically lies in a range between 150 bar and 500 bar, and in the case of diesel as fuel, said system pressure typically lies in a range between 1500 bar and 3000 bar. Pressure regulation by acquisition of a signal of the high-pressure sensor, processing of the signal by means of the control device and alteration of the delivery power of the high-pressure fuel pump by means of the digital inlet valve is normally performed. The high-pressure fuel pump is normally mechanically driven by the internal combustion engine itself, for example by means of a camshaft.
In the described high-pressure fuel pumps with a digital inlet valve, faults may arise which lead to an undesirably increased delivery power of the high-pressure fuel pump. This may for example be caused by the inlet valve on the high-pressure fuel pump no longer being able to be fully opened or closed. It is for example also conceivable that, for example as a result of a spring breakage at a spring in the inlet valve, or further possible faults, the delivery power can no longer be controlled.
In such a fault situation, a volume flow for the high-pressure fuel pump is set in a manner dependent on the rotational speed of the internal combustion engine and the temperature prevailing in the fuel injection system. Here, said volume flow may be greater than the injection quantity of the at least one injector valve. For example, in a typical operating state, the so-called overrun mode of the internal combustion engine, no or only little injection is performed through the injector valve. Therefore, if the high-pressure fuel pump delivers an excessively large volume flow, an undesired pressure increase occurs in the fuel injection system.
To be able to deplete undesirably high pressures in the high-pressure region of the fuel injection system, it is common for a mechanical safety valve, a so-called pressure-limiting valve, to be provided on the high-pressure fuel pump, which valve can limit or restrict the pressure.
Typical p-Q characteristics of the pressure-limiting valve are configured such that a maximum pressure takes effect in the high-pressure accumulator, which maximum pressure exceeds the nominal pressures of the injector valve during normal operation.
After the fault situation, the pressure increases within a few pump strokes of the high-pressure fuel pump up to a maximum pressure, which takes effect in the high-pressure region.
The pressure-limiting valve is commonly designed so as to discharge into a pressure chamber of the high-pressure fuel pump, such that said pressure-limiting valve is hydraulically blocked during a delivery phase of the high-pressure fuel pump. This means that the pressure-limiting valve can open, and discharge fuel out of the high-pressure region, exclusively in the suction phase of the high-pressure fuel pump. Such pressure-limiting valves are referred to as hydraulically blocked pressure-limiting valves.
Owing to the structural nature of the injector valve, the injector valve commonly opens counter to the pressure prevailing in the high-pressure accumulator. Here, in a manner dependent on the operating state of the internal combustion engine, an actuation profile is used for the actuation of the injector valve in order to open the injector valve such that an injection can begin.
Many injector valves are designed not for the maximum pressure in the fault situation but, in a cost-optimized manner, for normal operation. In this way, in fault situations with excessively high pressures in the high-pressure region, the injector valve can no longer open, and the internal combustion engine can thus no longer operate. This can result in a breakdown of a vehicle operated with the internal combustion engine.